It wont switch on
I've tried replacing the power cord; plugging into the wall instead if the power bar; unseated and re-seated all the connects to the mainboard.

Could it be the power supply, or perhaps even the power on switch? A few days ago it was working just fine, then the following day it wouldn't switch on! A day later it did switch on but about 5 hours later it randomly shut off!

On this motherboard, the standby power LED denotes if there's power being received. Since that's not on, and you've checked all the cabling to ensure it's connected securely, I'm guessing the PSU has failed.

Here is another vid
You did turn the power switch back on to test? Are you sure your cord is good and your outlet has power? Make sure your voltage switch is set for what you have 110V or 220V. I had one that the voltage switch got dirty and had to move it back and forth to get it to work. (not while it's on)

First some observations;
1 Testing a pw supply with a paperclip is useless. It does nothing to test the unit under a load. A MUCH better test is to swap in a known good pw supply. Note known good does NOT mean new; it means a pw supply pulled from a working system that you know works. A pw supply can be DOA just like any other part and that will really mess up your troubleshooting
2 A cmos batter will not stop the system from pw ON. There will be errors ie time/date, no boot device, etc, etc however it will pw ON
3 Your motherboard is now 10yr old and [IMO] it is not worth putting any money into 10yr old tech. Up to you if you want to attempt to fix this system. The very first thing to do is swap in a known good pw supply for testing.